Solar cells have been developed for generating electrical energy directly from sunlight. In general, these cells can be classified as either heterojunction devices or homojunction devices. Heterojunctions are those formed between two different semiconductor materials or between a metal and a semiconductor or from a metal/insulator/semiconductor sandwich. Homojunction devices depend only upon junctions formed between layers of the same semiconductor material doped with different impurities to provide different electrical properties.
Heretofore, homojunction cells using direct-gap semiconductor materials have generally exhibited disappointing efficiencies. Direct-gap materials are strongly photon absorbing semiconductor materials from the III-IV group. One reason for the relatively low efficiencies in homojunction solar cells is believed to be the high absorption coefficient which is inherent in direct gap semiconductor materials such as gallium arsenide. For example, approximately half of the carriers due to AM 1 radiation are generated within 0.2 .mu.m of the surface of gallium arsenide. Therefore, for materials such as GaAs, which also has a high surface recombination velocity, most of the carriers generated by solar radiation recombine at the top layer before they reach the charge separation junction, causing a significant decrease in conversion efficiency.